Traditional three-way catalysts are highly effective at reducing NO and NO2 (“NOx”) to N2 in the exhaust of standard gasoline engines. Despite their overall utility, these catalysts are not effective at so called “lean burn” conditions, wherein the exhaust is substantially enriched in oxygen such as in a diesel exhaust or, less commonly, the exhaust from lean burn gasoline engines. At the time three way catalysts were developed, the inability to operate at lean burn conditions was not detrimental as very few engines operated under these conditions. However, as government fuel efficiency requirements have become more stringent, many car manufacturers have opted to use lean burn technology, thus necessitating the development of new catalytic systems capable of reducing NOx in an oxygen rich environment.
One catalyst capable of operating under lean burn conditions is the hydrocarbon selective catalytic reduction (“HC-SCR”) catalyst. The general reaction for the HC-SCR process is: 2NO+“H2C”+0.5O2→N2+H2O+CO2 wherein H2C is a hydrocarbon, such as for example, n-octane.
One HC-SCR catalyst system capable of catalyzing the conversion of NO to N2 under lean burn conditions is silver (Ag) supported on alumina. See, e.g., Shimizu, et al., “Selective Catalyst Reduction of NO over Supported Silver Catalysts.” Phys. Chem. Chem. Phys., 2006, 8, 2677-2695. Despite the ability of silver on alumina to reduce NOx under lean burn conditions, the silver/alumina catalyst system is not yet sufficiently developed or understood for deployment in vehicles. Moreover, attempts to optimize the performance of the catalyst have produced mixed results.
For example, in order to achieve the greatest reduction of NOx products using a silver/alumina HC-SCR catalyst, it may be necessary to vary engine conditions, resulting in reduced fuel efficiency in a given engine. Another shortcoming of the silver/alumina HC-SCR catalyst is its propensity to produce toxic byproducts such as NH3, CH3CN, and HCN. While these molecules can be scrubbed from an exhaust stream using supplemental catalysts, the need to introduce additional catalysts (such as Pt or Pd) increases the overall cost of a catalytic system.
Thus, what is needed is an HC-SCR catalyst capable of more efficiently converting NOx to N2 under a variety of real world operating conditions. The HC-SCR catalyst should likewise be capable of converting toxic byproducts produced during the HC-SCR reaction to non-toxic, non-polluting compounds or produce no toxic byproducts at all.